Running Gear Forces at High Speed

I was just watching videos on yoUtube showing large steam engines at high speed. I was absolutely amazed at how fast the rods (coupling, connecting, driving - I am not sure of the correct terminology) were moving. I know some of the rods are very big steel members with a lot of mass as I have seen them up close in museums. That is why I was so awed about how fast they moved. It must take a huge amount of force to accelerate the mass of these rods every time they change directions and they are continually doing so. Motion is in two planes. Each time a rod's horizontal component changes direction, with each half wheel rotation, the rod must be decelerated to a complete stop and then accelerate in the opposite direction. The same for the vertical component. I would think the bearings and steel structures holding the rods in place must be very prone to fatigue failures. Even with well lubricated and performing bearings the forces still have to be dealt with by something. How were they able to keep the running gear from constantly experience catastrophic failures?

maintenance, maintenance, maintenance, and more maintenance!!!

That is why steam was replaced by diesels.

Wheel counter weights and connecting pins being 90 degrees apart were attempts to smooth out the pounding of the drivers. It was close, but never perfect.

GSC wrote:Wheel counter weights and connecting pins being 90 degrees apart were attempts to smooth out the pounding of the drivers. It was close, but never perfect.

The counterweights certainly helped with the pounding but old Newton's second law can't be ignored. Every time the velocity (both direction and speed) of the massive steel drivers change, a force needs to be applied and that force is directly proportional to the mass of the steel and how fast you are trying to change its direction and speed. No matter how well lubricated or balanced, that enormous force must be born by the bearings that attach the drivers to the wheels etc. From the videos I recently saw, the drivers are moving so fast the forces must be huge. I am amazed the bearing points are not quickly work hardened from the enormous constantly changing cyclical stresses, become brittle, and quickly fail (especially in very cold weather). Obviously this did not happen but it is incredible to me these systems held together long enough to get a steam locomotive to its destination. This is definitely a compliment to the Mechanical Engineers and material guys that designed these systems 100+ years ago. Also, I would imagine it must have been a spectacular event, with the potential of significant harm to the engine crew, if one of the drivers did get loose.

Looking at the journals holding the drive axles to the frame, you can see just how heavy duty they were. Massive, as compared to the axles. Even old 4-4-0s had some seriously large journals, so the designers even back then knew of the forces needed to be dealt with.

There is a lot of reciprocating machinery in the world today. the necessary proportions of the various parts was worked out long ago.

the gas engine in your car is subject to exactly the same forces as the steam locomotive engine and operates at much higher rpm's, but no one thinks that's amazing.

Eliphaz wrote:There is a lot of reciprocating machinery in the world today. the necessary proportions of the various parts was worked out long ago.

the gas engine in your car is subject to exactly the same forces as the steam locomotive engine and operates at much higher rpm's, but no one thinks that's amazing.

How many internal combustion engines have moving parts as heavy as on a steam locomotive though, or driving rods as large? Lighter parts are easier to balance, and none of those internal-combustion motors are directly connected to whatever driving wheels their vehicles have. And IINM, a gasoline engine can't experience exactly the same forces as a steam locomotive, except for maybe older ones with saturated-steam boilers and no superheating; a gas engine usually compresses its fuel to about 185 psi, whereas pressures in steam locomotive pistons can be much higher (boiler pressures of 300 psi and steam pressures much higher than that entering the piston due to superheating the working fluid).

amtrakowitz wrote: How many internal combustion engines have moving parts as heavy as on a steam locomotive though, or driving rods as large?

lots of internal combustion engines are much MUCH bigger than any locomotive ever dreamed of being.
look at this 6 cylinder diesel, look at it!


look at this freekin crank shaft!

amtrakowitz wrote: Lighter parts are easier to balance, and none of those internal-combustion motors are directly connected to whatever driving wheels their vehicles have. And IINM, a gasoline engine can't experience exactly the same forces as a steam locomotive, except for maybe older ones with saturated-steam boilers and no superheating; a gas engine usually compresses its fuel to about 185 psi, whereas pressures in steam locomotive pistons can be much higher (boiler pressures of 300 psi and steam pressures much higher than that entering the piston due to superheating the working fluid).

the mean effective pressure of a steam engine is usually taken as 40% of boiler pressure. for a 270 psi boiler that's 108psi.
superheating results in lower not higher pressure in the cylinder due to friction loss.
mean effective pressure in non-turbocharged spark ignition engines , according to this wiki is in the range of 125-150psi, and in turbo diesels MEP is in the range of 200-270psi
so it appears both the speed and the impulse force is higher in internal combustion engines than ever was the case in steam locomotives.

large machine or small, engineers correctly size parts according to well understood relations to achieve specific service life expectations; and machinists build and balance machines to the standards achievable with the tools available to them.

Not to contradict anything Eliphaz says, but the engine components of a (conventional) steam locomotive do their work in "more trying circimstances" than the bits of most internal combustion engines in a couple of ways:
1) they are directly connected to the driving wheels that jiggle up and down and go THUMP at low rail joints and crossings
2) they are outside, exposed to the elements and to road grime, which has got to make it harder to keep them properly lubricated.

As Brian Rice said, "Maintenance, maintenance, maintenance and more maintenance!!!!" Slack off on that ... "Trains" had a story a while back about the late steam era (post WW II): one of the big streamlined CMStP&P Hudsons suffered catastrophic running gear failure. Author suggested this was because, with diesels on the way, maintenance had been cut back.

Hi all,

Great topic! Maintenance is certainly part of the reason that steam lomotives worked (and part of their downfall) but the best answer is that from a design standpoint, locomotive designers had a very good handle on the sizes of rods required, because they had a good idea of the forces being dealt with. Yes, the rods were very heavy, and valve gear components were too, but they had to be. Poppet valves, never really successful here in the USA, were a way to try to eliminate some of those problems with reciprocating valve gears, PLUS gain the advantages with rotary drive systems. But the short answer as to why they worked and didnt (usually!) fly apart begins with proper design from an engineering standpoint.

David A. Davis

Allen Hazen wrote: "Trains" had a story a while back about the late steam era (post WW II): one of the big streamlined CMStP&P Hudsons suffered catastrophic running gear failure.

I searched for more information on the Hudson running gear failure and was unable to find anything. Do you have a reference you might suggest? Thanks

BTW, although I started this thread, I think the obvious answer to my question is exactly what everyone has been saying; clever and well engineered equipment along with unrelenting maintenance. However, although I am an Electrical Engineer, I still have retained a lot of what I learned 40 years ago in second semester sophomore dynamics. Specifically Newton's second law and F=MA. As discussed, this means any change in speed or direction of those big massive steel drivers ultimately imposes incredibly huge forces on the bearing points. This probably means there were a number of very unique and very interesting engineered design features to make these systems reasonably reliable that we just do not now usually see in contemporary machinery. I guess that is really what I was looking to learn about in my initial post.

RhoXS wrote:incredibly huge forces

Must not be utterly incredible. If you're just looking at the crankpin in a driver, the inertia of the siderod is producing a constant force radially and its weight is producing a constant force downward, and the force of the steam is alternating forward and backward. Think it would be worthwhile to estimate the size of each force?

Well,
it's also feelable in the cars... if the steam engine is running on idle, the counterweights will help to prevent the motion forces of massive crankpins and rods... But if the engine goes into pulling force, we can simply feel to power of the masses. Because right now we can simply feel the powers, because the most engines start to create a swinging action forwards and backwards, resulting in a shrug of the train and the couplings.

So, most cranks and bearing can handle the load generated by the force, but the forces up and down of the wheels can be measured and can be huge on very fast traveling trains. If one engine has a axle load of 20 tons, it can reduce the axle load on top mark of the cranks to about 15 tons.
But, machinery of the cranks and rods isn't that problematic at all... all is constructed well to handle those forces.

So the most point is well said: Maintainance. Oil those parts well, pay attention to the bearing surfaces... and control the bearings, rods and mountpoint well and often, to prevent damage. This is the more important part.

Modern diesel engines, modern internal combustion engines have an automated oil supply to the moving parts, so the in our diesel or gasoline engines the cranks dip into the oil of the oil pan, so the bearing is still fresh with oil cooled and oiled. So it's not the water, what usually abandon the heat problem on those bearings, it's the oil, that's why modern engines have oil coolers, because they keep the moving parts of the engine cool and still in proper condition.
In our steam engines, this isn't possible. The main bearings usually have grease fillings, or have their own oil can. But... this can't be compared.
Still those bearing suffer from more heat, suffer from the forces, thus need more attention that modern engines. Thus:
Maintainance... it's the important part.